The State Department said attacks were possible "particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula," according to AFP.

"Current information suggests that Al-Qaeda and affiliated organizations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the State Department said in a worldwide travel alert for U.S. citizens.

The alert warned of "the potential for terrorists to attack public transportation systems and other tourist infrastructure."

As a precaution, the State Department said it was closing at least 22 U.S. embassies or consulates on Sunday, a work day in many Islamic countries.

The missions cover virtually all of the Arab world and also include two embassies in predominantly Muslim non-Arab nations -- Afghanistan and Bangladesh -- as well as U.S.-run facilities in Israel.

The missions posted notices on their websites saying that they would be closed on Sunday.

U.S. State Department spokeswoman Marie Harf said that some embassies or consulates may decide to remain closed after Sunday.

Nancy Pelosi, the leader of President Barack Obama's Democratic Party in the House of Representatives, said that congressional leaders received a briefing on the threat.

"There is some understanding of the seriousness of the threat," Pelosi told reporters, according to AFP.

Representative Jason Chaffetz, a Republican who has criticized the Obama administration over embassy security, said he understood there was "a very real worldwide threat."

"There are threats that are coming at us on a daily basis but this rises to a new level," Chaffetz told CNN.